Wireless LANs. ITS 413 Internet Technologies and Applications

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1 Wireless LANs ITS 413 Internet Technologies and Applications

2 Aim: Aim and Contents Understand how IEEE wireless LANs work Understand what influences the performance of wireless LANs Contents: IEEE Architecture and Physical layer IEEE Medium Access Control (MAC) Management Features Addressing Distributed Coordination Function Performance ITS Wireless LANs 2

3 IEEE Wireless LANs Concepts Architecture Physical Layer

4 A Quick Intro to Wireless LANs Wired LAN Wireless LAN Client Wireless LAN Access Point IEEE Wireless LANs define how to communicate between a Client and Access Point. From Access Point to other hosts is via any other standard (e.g. Ethernet, IP, ADSL) Therefore, IEEE is concerned with link communications, That is, Layer 1 (Physical) and Layer 2 (Data Link). Internet ITS Wireless LANs 4

5 How fast? A Quick Intro to Wireless LANs Most common standards are: 11Mb/s and 54Mb/s Throughput is around half (5 to 25Mb/s), but shared! How far? 10 s of metres indoor, 100 s of metres outdoor How much power? 1mW up to 100mW What frequency? Most common is 2.4GHz, an unlicensed band Are they secure? Satisfactory link level security is provided by WPA ITS Wireless LANs 5

6 Terminology and Concepts Clients Client associate with an AP Access Point (AP) A set of clients and the associated AP are called Basic Service Set (BSS) or cell ITS Wireless LANs 6

7 Terminology and Concepts A set of all clients and all APs within is called an Extended Service Set (ESS) A client can move between BSS within a ESS: the process of changing APs is called handover ITS Wireless LANs 7

8 IEEE 802 LAN Architecture IEEE 802 LAN/MAN standards committee develops standards following a common architecture ITS Wireless LANs 8

9 IEEE Wireless LANs IEEE wireless LAN standard comprises: Multiple Physical layer standards (original, a, b, g, n) Single MAC protocol standard, common across different physical layers Optional enhancements E.g. quality of service (11e), spectrum (11k, 11j), security (11i), 11 MAC 11 Phy 11a Phy 11b Phy 11g Phy 11n Phy IEEE Wireless LAN Standards ITS Wireless LANs 9

10 IEEE Physical Layer Remember: the Physical layer defines how to send 0 s and 1 s as a physical signal (e.g. radio waves) What does the IEEE Physical layer define? Modulation: shape the analog signal into efficient form Frequency: the part of frequency and bandwidth the signal is sent at Timing: how to synchronise sender/receiver What are the practical characteristics of different Physical layers? Data rate: the speed at which data is sent [bits per second] Transmission range: the maximum distance that two nodes can communicate [metres] Depends on transmit power and receiver sensitivity Frequency: determines how transmissions travel in an environment, and potential with interference with other sources [Hertz] Non-overlapping channels: the number of channels (frequencies) that can be used at the same time without causing interference ITS Wireless LANs 10

11 IEEE Physical Layers 11 11b 11a 11g 11n Released Frequency 2.4GHz 2.4GHz 5GHz 2.4GHZ 5GHz Modulation DSSS DSSS OFDM OFDM OFDM, MIMO Channels Data Rate 2Mb/s 11Mb/s 54Mb/s 54Mb/s 300Mb/s Range m m 15-30m 25-75m 20-60m ITS Wireless LANs 11

12 Broadcast Radio Normal operation of wireless LAN equipment is as broadcast radio (point-to-multipoint) Signal Receiver Transmitter The frequency bands used (2.4GHz, 5GHz) are unlicensed spectrum in most parts of the world Anyone can use the frequencies, without paying (however, they most follow some rules) Can easily lead to interference between devices ITS Wireless LANs 12

13 Characteristics of Broadcast Radio Some assumptions about broadcast transmissions: A radio device s transmission can be received by all other devices within the transmission range For example, if the transmission range is 30m, then all nodes within 30m of the transmitter will receive the transmission. A node that is 31m (or more) from the transmitter will not receive the transmission (its not that simple in practice though) The transmission goes in all directions: omni-directional A radio device cannot transmit and receive at the same time Due to interference, it is difficult to implement a transmitter/receiver (transceiver) that operate at same time Therefore, we have half-duplex operation (Whereas most wired networks today, such as Ethernet, allow transmission and reception at same time: full-duplex) A radio device cannot successfully receive transmissions from two or more sources at the same time The two transmissions interfere with each other, making it almost impossible for the receiver to determine what the two original signals were So we assume that if two (or more) devices transmit at the same time, then a receiver cannot successfully receive either of them. Often called a collision between packets ITS Wireless LANs 13

14 IEEE MAC Management Procedures

15 IEEE MAC Layer IEEE Physical layer provides means for sending data over wireless medium IEEE MAC layer: Defines management procedures for discovering, joining and leaving BSS/ESS How does your laptop find an AP? How does it connect to the AP? Defines protocol for efficient and robust communication over wireless medium How does your laptop share access with other surrounding laptops? Is common across different physical layers Same MAC used for 11a, 11b, 11g (although some parameter values change) Uses Hardware (MAC) addresses to identify clients/aps The format of Hardware addresses is same as Ethernet: 48-bit addresses ITS Wireless LANs 15

16 IEEE MAC Management When you turn on your laptop, it needs to find an AP to associate with. How does it find it? Two approaches are used: APs periodically broadcast Beacon frames advertising itself Any client that receives the Beacon frame will know that the AP exists (and can choose to associate with the AP) A client can broadcast a Probe Request frame, searching for an AP Any AP that receives the Probe Request frame may respond with a Probe Response frame The client, upon receiving the Probe Response, can choose to associate with the AP Once your laptop knows about an AP, how does it associate? Authentication Allows AP to check whether client is allowed to connect to network Association Registers the client with the AP Once associated, a client and AP can exchange data. When the client shuts down or moves, what happens? Either client or AP can de-authenticate or disassociate If moving from one AP to another AP, a special re-association can occur There are ways to make the handover fast ITS Wireless LANs 16

17 MAC Management Discovery Client Beacon AP Periodic broadcast by AP; Probe request/response: active discovery by client Joining Leaving Authenticate Req. Authenticate Resp. Associate Request Associate Response Disassociate Deauthenticate Re-association possible. Moves association from AP to AP Either client or AP can disassociate and/or deauthenticate. ITS Wireless LANs 17

18 MAC Management Frames Discovery Frames Beacon: periodic broadcast by AP (e.g. 10/sec) Probe Request: active discovery by client Probe Response: APs response to Probe Request Joining Frames Authenticate Request (client) / Response (AP) Associate Request (client) / Response (AP) Reassociate Request (client) / Response (AP) Leaving Frames De-authenticate (client or AP) Disassociate (client or AP) (these are notifications, not requests; no responses needed) ITS Wireless LANs 18

19 IEEE MAC Data Transfer

20 IEEE MAC Data Transfer The aim of MAC is to provide efficient, robust and fair data transfer Efficient: minimises overheads in the bandwidth scarce wireless medium Robust: can handle errors Fair: all stations get an equal share of the wireless medium Remember: operate in a broadcast radio environment. If two nodes transmit at the same time, then collision (lost data). Therefore aim of MAC is: Only one node transmits at a time There were two approaches for transferring data in the original IEEE standard: DCF and PCF Today, DCF is most commonly used (along with enhancements developed in IEEE e etc.) Distributed Coordination Function (DCF) Carrier Sense Multiple Access (CSMA) Collision Avoidance (CA) DCF is mandatory in Point Coordination Function (PCF) Polling based transfer: the AP indicates a schedule for the clients to transmit ITS Wireless LANs 20

21 Distributed Coordination Function Distributed Coordination: Aim is that only one node transmits at a time Each node implements the function, so that they determine when they are allowed to transmit in a distributed manner In DCF, clients and APs are the same Lets refer to them as stations Two modes of operation: Basic Access mode RTS/CTS mode Depending on the size of the Data to transmit, the mode will be chosen For a smaller Data, use Basic Access; for larger Data use RTS/CTS ITS Wireless LANs 21

22 DCF Frames DATA ACK RTS CTS Sent from Sender to Receiver Data (or payload) up to 2312 bytes (typically limited to 1500 bytes, and often varies) Header+Trailer: typically 34 bytes Acknowledgement from Receiver to Sender Typically 14 bytes Request To Send, from Sender to Receiver Typically 20 bytes Clear To Send, from Receiver to Sender Typically 14 bytes Header Data Trailer Ack RTS CTS ITS Wireless LANs 22

23 Frame Control DATA Frame Bytes: Duration A1 A2 A3 Sequence Control A4 Frame Body FCS Used to update NAV (see RTS/CTS) 12 bit sequence number 4 bit fragment number Addresses (A1-A4) depend on direction of frame Source Address, e.g. client address Destination Address, e.g. LAN client BSSID: AP address (A4 for wireless bridging) Frame body: up to 2312 bytes (maximum for Ethernet 1500) 32 bit CRC for error detection ITS Wireless LANs 23

24 DATA Frame Control field Protocol Version Type Subtype To DS From DS More Frag Retry Pwr Mgt More Data WEP Order Field Bits Description Protocol Version 2 Value: 0 Type 2 Control, data, management Subtype 4 Probe Req., Data, Ack, To DS 1 From DS 1 00: Ad hoc; 10: Client to AP; 01: AP to client; 11: AP-AP (bridge) More Frag 1 More fragments to follow Retry 1 Retransmission Pwr Mgt 1 Power save mode More Data 1 Power save or CFP WEP 1 On or Off Order 1 StrictlyOrdered/OrderableMulticast ITS Wireless LANs 24

25 Addressing in There are four address fields that can be carried in a frame Each carry IEEE 48-bit Hardware addresses Three of the four addresses are commonly used: Client address: the client involved in the transfer AP address: the AP involved in the transfer LAN address: the client on the other LAN involved in the transfer ITS Wireless LANs 25

26 Addressing in To LAN A2 A1 A3 A1: Access Point A2: Source WLAN Client A3: Dest. LAN Client ITS Wireless LANs 26

27 Addressing in From LAN A1 A2 A3 A1: Dest. WLAN Client A2: Access Point A3: Source LAN Client ITS Wireless LANs 27

28 Addressing in To WLAN A2 A3 A1 A1: Access Point A2: Source WLAN Client A3: Dest. WLAN Client ITS Wireless LANs 28

29 Distributed Coordination Function

30 DCF Basic Access Concept: A station will transmit if the medium is idle (no-one else transmitting) If medium is busy (someone else transmitting), then wait until they have finished and then transmit Operation: When station has data ready to send: 1. Medium must be idle for period of DCF Inter Frame Space (DIFS) 2. After DIFS, medium must be idle for Backoff period 3. When backoff complete, transmit DATA frame 4. Upon receipt of ACK frame, data transfer is complete If medium becomes busy during DIFS: Wait until idle, then restart from point 1 above If medium becomes busy during Backoff: Suspend backoff counter, wait until idle, then restart from point 1 above Continue backoff from where it was suspended ITS Wireless LANs 30

31 DCF Basic Access Interframe Spaces DCF IFS (DIFS): period that the medium must be sensed idle before starting backoff Short IFS (SIFS): period to wait between frame transmissions during data transfer E.g. Receiver waits SIFS before sending ACK SIFS is always less than DIFS Backoff Period R = random integer between 0 and CW Backoff Period = R x SlotTime CW: Contention Window size, initially CWmin Choosing a random Backoff period minimises collisions after two or more stations defer Provides fair access to all nodes (on average, every station gets same chance of winning access) Parameter values are defined for each Physical layer DIFS, SIFS, SlotTime, CWmin, CWmax ITS Wireless LANs 31

32 DCF Basic Access Example A B C A has data to send to B: the medium is idle for DIFS plus Backoff period, so sends the DATA and receives an ACK C has data to send to B: the medium becomes busy during DIFS, so defer until idle again. Finally, after medium is idle for DIFS, C selects a Backoff B has data to sent to A: the medium is idle for DIFS, selects a Backoff In this example, it assumes B completes its Backoff before C (that is, B chose a smaller random number). Therefore B transmits the DATA, while C has to defer again Finally, after B is complete, C finishes its Backoff and transmits the DATA ITS Wireless LANs 32

33 DCF Error Control After sending a DATA frame, a station waits for an ACK If no ACK received within ACKTimeout, then assume an error Double CW (up until CWmax) Retransmit the DATA frame, applying the normal rules ACKTimeout: assume it s the time for SIFS + ACK If too many retransmissions of a DATA frame are attempted, the station will abort and return an error to the user Too many is usually 7 (but can be modified) ITS Wireless LANs 33

34 DCF Contention Window The size of the contention window (CW) determines how long a station waits until it transmits (backoff) Backoff Period = R x SlotTime, where R is random number between 0 and CW For each new data frame, CW = CWmin For a retransmission, CW is doubled (up until CWmax) Example: CWmin = 31, CWmax = R is randomly chosen between 0 and CW The figure shows CW for a retransmitted frame Original Transmission 1 st Retry 2 nd Retry 3 rd Retry 4 th Retry 5 th Retry 6 th Retry 7 th Retry ITS Wireless LANs 34

35 DCF Contention Window Purpose: If two stations want to transmit, then which one does? By selecting a random time to backoff, in most cases, one station will choose a shorter backoff than the other The station with the shortest backoff transmits first (the other will have to wait). This implements the fairness the chance a station gets to transmit is random (on average, all station with have equal chance to transmit) But what if the two stations choose the same random number? They both transmit and collision occurs The larger the value of CW, the less chance two stations will choose the same random number. Therefore, less chance of collision If a collision does occur, then CW is increased (doubled) so there is even less chance of a collision for the retransmitted frame However, the larger the value of CW, the longer time spent in backoff. Therefore, less efficient ITS Wireless LANs 35

36 A Problem: Hidden Stations Consider the scenario below: Client A AP Client B Client A is within range of AP Client B is within range of AP Client A is out of range of Client B (or Client A is hidden from Client B) DIFS ACK Timeout DIFS ACK Timeout DIFS A Backoff DATA Backoff DATA (Retransmit) Backoff Data to AP AP Collision at AP! Collision at AP! DIFS ACK Timeout DIFS B Backoff DATA Backoff DATA (Retransmit) Data to AP

37 A Problem: Hidden Stations When two stations are hidden from each other (e.g. out of range, cannot hear each other), then very high chance of collisions at a receiver (e.g. AP) Collisions lead to retransmissions; even chance of further collisions on retransmissions Retransmissions lead to significantly lower throughput (takes a long time to successfully send a single DATA frame) This is called the hidden station (or terminal) problem A solution: Before sending the DATA frame, ask the receiver if they are busy (Request To Send) The receiver responds if they are not busy (Clear To Send) If receiver is busy (receiving from someone else), then will not respond When sender receives the Clear To Send, they send the DATA frame In IEEE DCF there are two modes of operation Basic Access: DATA then ACK RTS/CTS: RTS then CTS then DATA then ACK ITS Wireless LANs 37

38 Basic Access and Hidden Stations There is a high probability of collisions if using Basic Access in the presence of hidden stations. Client B starts transmitting because it believes the medium is idle (because Client B cannot hear Client A s transmission) ITS Wireless LANs 38

39 RTS/CTS and Hidden Stations Using RTS/CTS before sending DATA reduces the probability of collision in the presence of hidden stations. When the AP responds with a CTS, Client B hears the CTS and immediately defers because someone else is about to transmit. ITS Wireless LANs 39

40 RTS/CTS Frames RTS Frame Request To Send Sent to intended recipient of DATA Notifies all stations of upcoming DATA frame Size ~ 20 bytes CTS Frame Clear To Send Response from the recipient of RTS Notifies all stations of upcoming DATA frame Size ~ 14 bytes DATA and ACK also used ITS Wireless LANs 40

41 RTS/CTS Operation Before sending DATA, a successful RTS and CTS handshake is needed Sender sends RTS to receiver: Request To Send Everyone else who hears the RTS is aware a transmission is about to take place they defer Receiver responds with CTS (if it is ready) Everyone else who hears the CTS is aware a transmission is about to take place they defer Sender sends DATA to receiver, and ACK is sent in response The same access procedures as Basic Access applied to sending RTS frames e.g. sense medium idle for DIFS then backoff SIFS before sending CTS, DATA, ACK All stations receiving RTS or CTS set their Network Allocation Vector (NAV) based on Duration field in the RTS or CTS frame NAV keeps track of when the medium is in use After the NAV period, other stations can attempt transmission (normal backoff rules apply) ITS Wireless LANs 41

42 RTS/CTS Example Client A AP Client C Client B Lets assume all clients are within range of the AP, but no within range of any other client (e.g. A cannot hear B or C, B cannot hear C) All clients have data to transmit to the AP ITS Wireless LANs 42

43 RTS/CTS Example DIFS DIFS SIFS DIFS SIFS SIFS SIFS SIFS SIFS SIFS SIFS DIFS SIFS Even though the clients are hidden from each other, the use of the RTS/CTS means each client becomes aware of the other clients transmissions (since each client hears the CTS from the AP). As a result there are no collisions: one station transmits at a time. ITS Wireless LANs 43

44 Basic Access vs RTS/CTS In practice, Basic Access is used for small DATA frames, RTS/CTS used for large DATA frames RTSThreshold is a parameter in wireless LAN devices Why? If the DATA frame is smaller than RTSThreshold, then use Basic Access If the DATA frame is larger than (or equal to) RTSThreshold, use RTC/CTS RTS/CTS is good for solving hidden terminal problem (and in general, avoiding collisions), but introduces an extra overhead With small frames, the overhead is very significant Other factors that determine RTSThreshold: Amount of traffic/nodes in network: with higher traffic it is better to use RTS/CTS more (hence lower RTSThreshold) Use of RTS/CTS may lead to exposed terminal problem, which reduces the throughput! ITS Wireless LANs 44

45 IEEE MAC Performance

46 IEEE MAC Performance Physical layer offers raw data rate (e.g. 54Mb/s in g) E.g. when station transmits a 1000 Byte frame using IEEE g it takes 148μs to transmit But not all time is spent transmitting frames MAC introduces overheads to provide addressing, reliability and management: Frame headers Control frames: ACK, RTS, CTS, Interframe spaces Backoffs Collisions and retransmissions We are interested in throughput at MAC layer MAC layer throughput is the rate at which user data is successfully delivered to destination An approximate way to measure the throughput is to count the total data (minus MAC headers) delivered over a period of time 46

47 IEEE MAC Parameters Parameter b a g DIFS 50μs 34 μs 28 μs SIFS 10 μs 16 μs 10 μs SlotTime 20 μs 9 μs 9 μs CWmin CWmax ITS Wireless LANs 47

48 Simple Throughput Calculation Assume 1500 byte payload using Basic Access, no collisions or deference! Best case for g (54Mbit/s) Time = DIFS + AverageBackoff + DATA + SIFS + ACK Time = *9 + ( )*8/ *8/54 = 334 usec Throughput = 1500bytes/334 usec = 35.93Mbit/s RTS/CTS: need to add 2 x SIFS + RTS + CTS time 11g throughput: 33.61Mb/s But we need to consider collisions and deference! ITS Wireless LANs 48

49 Other Issues in Wireless LANs

50 Security in Wireless LANs Original Authentication Ensure the client has permission to access the network Originally used a shared secret key (Wired Equivalent Privacy, WEP) Client and AP must be pre-configured with the same secret key Confidentiality Ensure the communications between client and AP cannot be overheard WEP shared secret key also used for encryption WEP has several limitations In practice, if an attacker can collect several GB of traffic between a client and AP, it can discover the secret key Enhanced Wireless LAN Security Wireless Protected Access (WPA): increase key size and solve WEP problems IEEE i: complete security architecture that can use other network security mechanisms ITS Wireless LANs 50

51 Wireless LAN Design Issues How many users per Access Point? Performance per user drops as number of users increase But we want to minimise number of APs Costly devices, costly to install and manage Handover between APs may become inefficient Basic Access versus RTS/CTS What RTS threshold? Basic Access is more efficient if few collisions (unless hidden terminals) RTS/CTS helps avoid hidden terminals How to cover a large area? Cellular coverage: many small cells or a few large cells? Avoid interference between cells Use different frequencies, but only 3 non-overlapping frequencies available ITS Wireless LANs 51

52 Wireless LAN Design Issues How do we secure the network? Need to authenticate users (usually to a central network authentication server) Need encryption: Layer 2 ( WEP, WPA, 11i) or other layer (IPsec, VPN, ) How do we give priority to users and applications? Voice calls get priority over data traffic Quality of service management on APs; but what about network wide? ITS Wireless LANs 52

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